Submerged ice marginal forms in the Celtic Sea off Harbour, Implications for understanding regional glaciation and sea level changes following the last glacial maximum in Ireland

Colman Gallagher1, Gerry Sutton2 and Trevor Bell3 1Department of Geography, National University of Ireland, Dublin (UCD) 2Coastal and Marine Resources Centre, University College Cork 3Geography Department, Memorial University of Newfoundland

ABSTRACT This paper presents the results of acoustic surveys and video imaging of the seabed off , beyond a previously identified submerged palaeochannel. This system extends south-west into waters at c. -56 m OD and terminates in an area of possible glacigenic sediments. The aim of the new sur- veys was to obtain multibeam sonar imagery and to correlate it with seismic pro- files in order to understand the genetic relationships between the morpho-sedi- mentological elements of the seabed in this area. The multibeam sonar imagery revealed four arcuate, morphologically complex features that on-lap the regional bedrock and confine the previously identified palaeochannel. Both the morphol- ogy of these forms and their sedimentology, as revealed by video imaging, imply formation in an ice-marginal environment. These forms are hypothesised to reflect several stages of sedimentation at the margins of ice progressively retreat- ing from the nearshore shelf of the Celtic Sea during the Last Glacial Maximum.

Key index words: Multibeam sonar, seismic profiling, video imaging, ice margin, continental shelf. Introduction and aims

This paper presents the results of a multibeam sonar and shallow seismic survey of the seabed off Waterford Harbour, Ireland, carried out aboard the RV Celtic Voyager of the Irish Marine Institute. A previous seismic and multibeam survey (Survey 1) revealed a submerged, infilled palaeochannel extending from the mouth of Waterford Harbour south-west into the Celtic Sea to a depth of c. 60 m below present sea level ( tidal datum, Gallagher, 2002). Seismic imagery indicated that the palaeochannel lies between bedrock walls and is filled with deep sediments that, near the mouth of Waterford Harbour, contain inset units of stratified facies. The latter are at least 15 m deep and may reflect the transition from glacio-isostatically dominated to glacio-eustatically dominated sea level change (Gallagher, 2002). Seismic profiles of the palaeochannel reach in the deepest water revealed a change in seismic stratigraphy from the stratified sediments further inshore to an unjointed, massive substrate with a hummocky surface expression. This change in the stratigraphy was interpreted to reflect either a change in bedrock characteristics or the presence of consolidated glacial sediments associated with a submerged ice margin c. 56 m below present sea level (Gallagher, 2002). The geometry of the palaeochannel fill indicated adjustment to discharges of between 7297 m3.s-1 to 13378 m3.s-1 (Gallagher, 2002). However, the entire rock-cut palaeochannel cross-section in the same zones may reflect discharges of up to 47640 m3.s-1. While these

Irish Geography, Volume 37(2), 2004, 145-165. 146 Gallagher, Sutton and Bell

Figure 1: Locations off Waterford Harbour of originally inferred palaeochannel, glaciofluvial morpho-sedimentary zones and new multibeam and seismic imagery of submerged arcuate forms shown in Figures 5,6,7, 9 and 10. Submerged ice marginal forms in the Celtic Sea 147 discharge estimates may be subject to large errors (Maizels and Aitken, 1991; Rotnicki, 1991) they still represent a hydraulic discharge regime up to two orders of magnitude greater than present maximum flood discharges through Waterford Harbour (EPA, 1997; Gallagher, 2002). This very large hydraulic discrepancy supported the interpretation of the submerged palaeochannel as glaciofluvial in origin, present day river catchments in south-eastern Ireland being incapable of supplying discharges of the magnitudes indicated by the palaeochannel geometry. Finally, when the palaeomeander geometry of Waterford Harbour itself was examined, a coherent pattern of wavelength emerged between the relict fluviatile features of the harbour and the submerged palaeochannel beyond (Gallagher, 2002). Together, the seismic imagery and retrodicted hydrology provided strong evidence of a genetic linkage between the onshore and offshore glacial palaeo-environments of Waterford Harbour, including the location of a former ice margin, now submerged, and the identification of a morpho-sedimentary sequence reflecting the transition from glacio-isostatically dominated to glacio-eustatically dominated sea level change in the region (Gallagher, 2002). It was in this context that a new survey was undertaken in 2002 (Survey 2). The aim of this survey was to obtain multibeam imagery of the hummocky seabed in order to understand both its morphological characteristics over space and to determine its morphostratigraphic relationship with the previously determined bedrock geography of the area (Gallagher, 2002). Arising from these data, the hypothesised ice marginal status of the hummocky zone could then be tested and its genetic relationship with the palaeochannel determined. In addition to the new data from Survey 2, previously unpublished seismic imagery obtained in Survey 1 will be used here to aid both the characterisation and interpretation of the hummocky seabed.

Research context

Regional to local Quaternary sea level has varied as a consequence of the interaction between both glacio-eustasy and glacio-isostasy, the latter determining the elevation of the earth’s crust due to crustal loading by glacial ice and resulting mantle rheology. Hence, in formerly glaciated areas, regional to local sea level histories are complex over both space and time and may vary in sense relative to the absolute volume of water contained in the ocean basins. In Ireland, the dominant view of Pleistocene sea level has been one of low relative sea level correlating with low glacio-eustatic sea level. For example, Orme (1964), using civil engineering records, described the submerged glaciofluvial sediments of the Blackwater Valley near Youghal and determined that the glaciofluvial palaeochannel was adjusted to a sea level 70 m below that of the present. Similarly, Farrington (1959) intercorrelated the glaciofluvial terraces and bedrock profile of the River Lee near Cork and determined that the longitudinal profile of the bedrock palaeochannel reflected a glacial sea level lower than 45 m below present sea level. Stillman (1968), described submerged lacustrine freshwater silts and peat from Bantry Bay, county Kerry, and from them inferred a late glacial (Alleröd) sea level low stand at a minimum of 55m below present sea level. Other researchers have described submerged peats in the southern Irish Sea region, indicative of a late glacial low stand of at least 45m below present sea level (Eyles and McCabe, 1989; Devoy, 1995). The inferred sea level fall in the Waterford Harbour region of at least 56m (Gallagher, 2002) is, therefore, consistent with the 45m to 70m range of sea level fall found in these southern coastal regions. However, the reinterpretation of Irish Pleistocene sedimentary systems occupying the margins of marine basins and embayments as glaciomarine in origin (Eyles and McCabe, 1989; McCabe, 1993) brought into focus the assumptions made concerning the relationship 148 Gallagher, Sutton and Bell between the terrestrial and marine environments during glaciation. In particular, this body of work provided evidence of glacio-isostatic submergence of some glaciated margins during periods of low eustatic sea level and demonstrated that a single model of sea level response to glaciation may not be universally applicable around the Irish coast. Furthermore, work on the genesis of Ireland’s lowland glacial landform assemblages demonstrated a greater complexity over both space and time than had been assumed, some of the research suggesting a genetic link with both changing relative sea level and Heinrich Events (Eyles and McCabe, 1989; McCabe and Clarke, 1998; McCabe et al., 1998). An important implication of all these lines of research taken together is that sea level response to glaciation in Ireland is likely to have been regionally and temporally complex and that empirical data from around the Irish coastline are required if a better understanding of that complexity is to arise. In particular, such research must investigate the variation in regional styles, thickness and extent of glaciation on the Irish continental shelf. Study area Waterford Harbour and the nearshore shelf beyond it were chosen as the study area to complement ongoing research into the glaciation of the Barrow valley, which becomes confluent with the Suir valley 5.4 km above Waterford Harbour (Figure 1). Extensive Pleistocene glaciofluvial deposits along both the Barrow, especially north of , and the Nore, which is confluent with the Barrow, indicate that the catchment was a major conduit for the meltwaters of ice caps occupying the midlands of Ireland (Gallagher, 1997). Owing to long term tectonically controlled patterns of uplift and incision (Orme, 1964; Densmore, 1998) the Barrow-Nore meltwaters must have entered the marine environment through Waterford Harbour. In addition, it is likely that meltwaters from lowland ice moving eastward from east Cork (Orme, 1964) passed through the Suir valley and were confluent for a time with effluxes emanating from the Barrow-Nore catchment in the region of Waterford Harbour. Therefore, both the pattern of lowland glaciation and the small degree of freedom imparted on regional drainage by the pattern of Tertiary uplift suggest that Waterford Harbour and the shelf beyond it were the focus of major glaciofluvial discharges. In addition, coastal exposure in parts of Waterford Harbour and Bannow Bay (Figure 1) reveal a sequence of cold climate sediments including till (Culleton, 1978) overlying an OI Stage 6 to Stage 5 palaeo-beach sequence (Gallagher and Thorp, 1997) atop a bevelled marine platform (Wright and Muff, 1904; Martin, 1930; Orme, 1966). This stratigraphy indicates that inland glacial ice transgressed the present shoreline for a time during the last glaciation but provides neither a complete glacial genetic record nor a full record of relative sea level change during the last glaciation (Devoy, 1995).

Methods Owing to the need to understand the three-dimensional characteristics of the seabed in the area of interest, both shallow seismic sub-bottom profiles and multibeam sonar bathymetric (topographic) imagery of the seabed were obtained. Video imaging of seabed transects and grab samples of seabed sediments provided ground truth for the geophysical data. Sub-bottom profiling Sub-bottom profiling was done with a GeoAcoustics GeoChirp system, used in high- resolution mode at 1.5-11.5kHz, the towfish being ‘flown’ from a stern-mounted A-frame. Two channel signal processing (time variable gain and ramp delay) was done in real-time. This provided simultaneously a 130ms window, comprising the total range (i.e. the maximum Submerged ice marginal forms in the Celtic Sea 149 time waited for a reflected wave to be received and, therefore, equal to twice the distance travelled by a wave in time units), and a shorter sub-range, in effect giving images at two vertical scales. Data were displayed in real-time on a SVGA monitor and were recorded using both a thermal plotter and digitally on exabyte tapes. Ship position, speed and heading were obtained from the on-board twin Trimble GPS via the onboard Data Acquisition System (DAS). These data were recorded both manually, relative to the time-stamps printed automatically on the thermal plots, and digitally in the data log of the DAS. Thermal plots of the sub-bottom profiles were digitally scanned and processed in Corel Photo Paint post-cruise. Firstly, the vertical scale of the profiles was exaggerated to resolve better the different stratigraphic units encountered. Secondly, the profiles were contrast stretched to enhance differences in the acoustic properties of the stratigraphic units, visible as gray scale variation in the imagery. Water depths, determined from the two-way travel time to the seabed and measured sound velocity profiles (see below), are given in the text corrected to Dunmore East tidal datum (at +0.015m relative to Poolbeg datum).

Multibeam sonar imaging

The multibeam sonar used was a Kongsberg Simrad EM1002. This comprises a retractable, hull mounted, 95kHz piezo-ceramic transducer array producing 111 beams and calibrated for depths ranging from 2m to 1000m. The maximum sector coverage is 150º and the maximum swath width is 7.5 times water depth. Bathymetric accuracy (reproducibility) is 0.15m or 0.25% water depth. In order to offset the effects of sea state upon the vessel, pitch and roll calibrations were carried out prior to each survey. The transducer was linked via an onboard transceiver unit to a Sun workstation integrated with the ship’s twin GPS. All data were digitally recorded on DATs and displayed in real-time in sun illumination mode. Data were gridded into 7m bin dumps on board, post-processed to remove tidal anomalies and to georectify the imagery and finally handled as GeoTIFF images in Photo Paint. Video imaging Underwater video of the seabed was recorded with a sled mounted, towed video camera. This set-up provided continuous video imagery of 1 km transects, definitively ground-truthing the multibeam imagery of key seabed areas. Video signals were encoded with DGPS allowing the video images to be accurately positioned. Three single video frames from one transect have been extracted for this paper and are shown in Figure 8. Sound velocity profiling This was done using an AML sound velocity probe that transmitted data in real-time to a PC workstation onboard via a soft data cable. The probe was suspended 1m below the surface for 300s to prevent thermal shock of the electronics before being released to the bottom. After editing-out data of both the acclimatisation period and the upward journey, the sound velocity profile was transferred from the PC workstation to the EM1002 software in the Sun workstation via the ship’s LAN. Ancillary data The onboard DAS provided a combined navigational and environmental log, including time stamped position, heading, speed, seawater temperature and salinity. These data were accessed post-survey via Microsoft Excel. 150 Gallagher, Sutton and Bell

Survey organisation This cruise of 2002 (Survey 2) was intended to complement and add strategic detail to data collected in Survey 1 of 2000 (Gallagher, 2002). Survey 1, leg 1a (2000) consisted of a GeoChirp shallow seismic search for palaeochannel cross-sections off the entrance to Waterford Harbour between and Brownstown Head (Figure 1). Water depths in this area ranged from c.10 m to c.30 m. Twelve parallel survey lines oriented 140º-320º (geographic) and separated by 1 km were run, comprising a total survey-run of c.110 km cruised at between 5.5 knots and 7 knots. Survey 1, leg 1b (2000) was run in deeper water with two line sets, one parallel to those in leg 1a but with 2 km spacings, the other at 1.8 km separations oriented east-west. These subsets together comprised 11 lines, totalling c. 157 km cruised at between 6 knots and 7 knots and included EM1002 multibeam sonar imaging, albeit with no line overlaps. Survey 1, leg 1c (2000) comprised a detailed multibeam and sidescan survey of the seabed between Hook Head and Swine Head. A series of 8 overlapping, east-west oriented parallel swaths was surveyed. This provided complete multibeam and sidescan images of a 5.8 km2 area of seabed. Survey 2 (2002) was a multibeam sonar survey of Zone 3 comprising a box with corners at 52º03.5’N / 07º15.2’W, 52º03.5’N / 06º53.3’W, 51º54.6’N / 07º15.2’W and 51º54.6’N / 06º53.3’W and containing east-west oriented survey swaths. This provided coverage of approximately 530km2 area of seabed in this zone (Figure 1).

Results

The seismic imagery confirms both the presence of a palaeochannel emerging from Waterford Harbour and running to the south-west between bedrock walls and that the palaeochannel is filled with deep sediments beneath a thin drape of recent sediments. The seismic stratigraphy of the deep fill comprises a three-fold facies transition from near shore to deeper water (Figure 1): firstly, wide channelised cross-sections containing a sedimentary fill, itself punctuated by smaller channels filled with distinctly stratified sediments (Figure 1 Zone 1 and Figure 2); secondly, laterally differentiated channelised cross-sections characterised by massive to crudely stratified sediments (Figure 1 Zone 2 and Figure 3); thirdly, seismic sections characterised by a massive facies with a hummocky surface expression and bench-like features descending in elevation (Figure 1 Zone 3 and Figure 4). The hummocky surface and apparent lack of jointing in the substrate in Zone 3 are central to this research in suggesting that this zone is characterised either by bedrock of a different structure from that further inshore or by consolidated glacial sediments representing a submerged ice margin (Gallagher, 2002). In Zone 3, the multibeam imagery reveals two parallel, arcuate, morphologically complex features (Figure 5, Outer Arc West and Outer Arc East). Both arcs trend generally north-west to south-east but are concave to the east-north-east at their northern extremity. The arcs are only thinly draped by recent sediments and do not appear to be bedrock forms as they clearly onlap the bedrock and display none of the structural features associated with the bedrock. Also, structural elements of the bedrock can be seen to be continuous but buried by the arcs (Figures 6 and 7). This interpretation of the arcs in general is supported by the video imagery of Outer Arc East (Figure 8). The video frames reveal that the seabed surface both east and west of the arc consists of fine pebble gravels and sands. However, the arc itself is composed Submerged ice marginal forms in the Celtic Sea 151

Figure 2: Characteristic seismic profiles in Zone 1 (10 ms vertical scale bars = c. 7.5 m through sea water). 152 Gallagher, Sutton and Bell

Figure 3: Characteristic seismic profiles in Zone 2 (10 ms vertical scale bars = c. 7.5 m through sea water). Submerged ice marginal forms in the Celtic Sea 153

Figure 4: Characteristic seismic profiles in Zone 3 (10 ms vertical scale bars = c. 7.5 m through sea water). 154 Gallagher, Sutton and Bell Submerged ice marginal forms in the Celtic Sea 155

Figure 5 (left): Multibeam sonar image of submerged landform assemblage in Zone 3. North is to the left, grid size = 7.00 metres. Top left corner = N52°03’30.38” W07°15’01.46”. Note: bedrock structures visible beneath northern extremities of Outer Arcs, especially Outer Arc East; bedrock structures trending NE-SW between the southern limbs of the arcs; and small circular to elliptical structures on the arc crests. Grid size is 7m.

Figure 6 (above): Multibeam sonar image showing detail of Outer Arc East. Note that the northern extremity of the arc onlaps the exposed bedrock. Note also the circular and half-elliptical structures northwest of the pronounced recurve at the southern extremity of the arc segment. Grid size is 7m. For scale refer to Figure 5. 156 Gallagher, Sutton and Bell

Figure 7: Raw multibeam sonar image showing detail of Outer Arc West (right top quadrant of image). Note the closed, elliptical structure overlying exposed bedrock and the consistent concave- eastward curvature of both the arc segment and its individual components. Grid size is 7 m. For scale refer to Figure 5. of mutually supported, sub-angular, faceted boulders. The boulders are stacked on their a/b axial planes and are aligned parallel to each other. These textural and fabric characteristics imply deposition by an extremely energetic, unidirectional fluid flow and are consistent with the sedimentological properties of ice marginal to ice-proximal glaciofluvial environments. Inside and parallel to the laterally confining arcs, but at a lower elevation, are two less clearly defined arcuate forms (Figure 5, Inner Arc West and Inner Arc East). These appear to be bridged at their northern end by hummocky seabed (Figures 5 and 9), suggesting that they are a single, sharply recurved feature. Seismic imagery of Outer Arc West, and the northern end of both Inner Arcs reveals large blocky units with hummocky meso-scale surfaces and Submerged ice marginal forms in the Celtic Sea 157

Figure 8: Selected video frames of Outer Arc East. Coarse sand and fine pebble gravel seabed east of the arcuate feature (upper). The arc itself is composed of large, faceted, sub-angular boulders resting on their a/b axial planes (lower). 158 Gallagher, Sutton and Bell

Figure 8: Selected video frames of Outer Arc East. Above shows gravelly seabed west of the arc. massive substrates (Figure 10). The blocks are separated by subtle convex-upwards partings that extend to the seabed and dip south-eastward (Figure 10, seismic section 7/7). Hence, the south-eastern half of Outer Arc West appears to be composed of discreet sedimentary elements, stacked en echelon in a mutually off-lapping depositional sequence that youngs to the south-east, along the apparent longitudinal axis of the arc (Figures 7 and 9). It is hypothesised that altogether the forms comprising Zone 3 represent a submerged ice margin or glaciomarine limit associated with the last glacial maximum in Ireland. However, even if this interpretation is valid, it is still not clear if these forms were produced by inland ice flowing southward onto the continental shelf or by ice originating in the Irish Sea (Figure 11) (O’Coffaigh and Evans, 2001). Alternatively, the disposition of both the till limit surrounding Waterford Harbour and the previously identified palaeochannel emerging from it (Figure 1) suggest that the submerged arcuate forms may represent an ice margin located at a major subglacial portal of a marine terminating ice sheet (Figure 12). In this scenario, the arcs may represent an enlarging ice-confined channel located between the portal and open water to the south. A similar interpretation of the terrestrial till limit onshore (Figures 1, 11 and 12) appears likely (Gallagher, 2002) irrespective of the interpretation of the submerged arcs.

Figure 9 (right): Multibeam sonar image of bridging segment joining Inner Arcs (bottom right quadrant of image). Grid size is 7 m. For scale refer to Figure 5. Submerged ice marginal forms in the Celtic Sea 159 160 Gallagher, Sutton and Bell

Figure 10: Seismic sections through the arcs (for location of seismics, see Figure 1). Submerged ice marginal forms in the Celtic Sea 161

Figure 11: Interpretation of submerged arcs as the eastward retreating margin of ice sheet originating in the Irish Sea and confluent with inland ice (cf Blackhall Till limit) close to the present shoreline. 162 Gallagher, Sutton and Bell

Discussion and hypotheses for further research

Terrestrial and submerged till limits The Blackhall Till was deposited by ice moving from the north-west across the present coast onto the continental shelf during the Midlandian glaciation (Culleton, 1978; Thomas and Summers, 1982, Gallagher and Thorp, 1997). The talweg long profile determined by Gallagher (2002) suggests that the submerged palaeochannel in Waterford Harbour fill off- laps the Blackhall Till. If this is substantiated, it means that both the till limit that crosses Waterford Harbour and the submerged palaeochannel represent a process continuum limited by a single base level. This morphostratigraphy, the post-OI Stage 5 (i.e. Midlandian) age of the Blackhall Till (Gallagher and Thorp, 1997) and the submergence of the off-lapping palaeochannel would together imply that the hypothesised glacial to glaciofluvial interface was active during a period of low relative sea level in the Midlandian glaciation. While it is possible that elements of the palaeochannel fill are subglacial in origin, it is likely that most of the fill is proglacial. If so, this implies that most of the fill post-dates the proposed ice margin in Zone 3 but predates the margin of the Blackhall Till crossing Waterford Harbour (Figures 1, 11 and 12). The Blackhall Till is the uppermost glacigenic unit in local coastal exposures (Culleton, 1978; Gallagher and Thorp, 1997). Therefore, the till limit represents a glacial margin active during the northward retreat of ice off the continental shelf. However, the Blackhall Till limit probably represents only the earliest ice margin to have stabilised above present sea level following the last glacial maximum (c. 22 kaBP to 18 kaBP), therefore postdating earlier ice margins now submerged. It is now proposed that the arcuate features characterising Zone 3 represent one such submerged ice margin in this sequence of shoreward retreat. This inference makes it important that future research determine the full stratigraphic relationship in Waterford Harbour between the submerged palaeochannel and any other submerged ice margins in the region, especially in deeper water to the south and in Bannow Bay to the east of the present study area, which may host ice marginal forms deposited by ice originating in the Irish Sea basin (Culleton, 1978; Synge, 1981; O’Coffaigh and Evans, 2001). Together, these data have the potential to indicate the minimum relative sea level attained during the operation of the palaeochannel and ice margin(s). Also, in conjunction with models of glacio- eustatic sea level change, it will provide a direct measure of the glacio-isostatic adjustment of this part of the Irish crust (Devoy, 1995). The submerged palaeochannel In the context of the Waterford Harbour palaeochannel itself, it is likely that the initial inference of a palaeochannel extending south-west across Zone 3 (Gallagher, 2002) is incorrect. Instead, the terraces or braid bars inferred in cross-sections 7/9 and 7/X by Gallagher (2002) appear to be elements of the regional structural geology and not associated with the palaeochannel. However, the prediction of an ice margin in this zone (Gallagher, 2002) appears to be supported. This revision suggests that Zone 2 of the submerged palaeochannel is at least spatially associated with the submerged arcuate forms and, as the retrodicted hydrology of the palaeochannel in Gallagher (2002) applied only to Zone 1 and Zone 2, the interpretation of the palaeochannel as being adjusted to very large glaciofluvial discharges appears strengthened. The correct interpretation of this morphological assemblage is, therefore, important in understanding the spatial organisation of glaciation on the Irish continental shelf, the relationship between terrestrial glacial and glaciofluvial processes, Submerged ice marginal forms in the Celtic Sea 163

Figure 12: Interpretation of submerged arcs and palaeochannel as representative of a disintegrating glaciomarine margin, due to un-roofing and widening of a subglacial portal, followed by evolution to an ice-confined channel with open water to the south. 164 Gallagher, Sutton and Bell glaciomarine processes and the pattern of sea level change from glacial maximum to early deglaciation in the area. Importantly, if the inference of a glacial maximum age glaciomarine limit at c. -60m OD proves correct, with a eustatic sea level at -160m to -200m, an isostatic depression of between 100m and 140m is required for this part of south-eastern Ireland. Hence, this would imply an ice mass of between 300m and 420m in thickness over the nearshore shelf and present coastline at the last glacial maximum, when Zone 3 was an active ice margin. Palaeochannel inset units The inset units of Zone 1 may also shed light on the evolution of the palaeochannel. The inset units comprise at least 15m of stratified seismic facies (see Figures 1 and 2) and are traceable within the main palaeochannel fill over 2.03km (Figure 1). Given the geometry of the inset deposits in relation to the surrounding palaeochannel sediments, it is clear that the inset units on-lap, and therefore post-date, the main channel fill. In doing so, they clearly fill a sharply incised, elongated notch within the main channel sediments. These characteristics suggest a number of hypotheses relating to the genesis of the sequence. Firstly, the inset units may have been deposited as a subglacial fill of subglacial channels incised in the pre-existing fill of the palaeochannel during a marginal readvance. Secondly, they may comprise relatively ice-distal sediments filling depressions in older ice-contact or ice-proximal glaciofluvial sediments, the depressions possibly related to a former location of lateral ice-margins. Thirdly, the inset sediments may represent deposition from a waning phase of discharge following a phase of deep glaciofluvial scour associated with an outburst flood and responsible for the incision of the elongated notches. Neither of these explanations requires significant changes in relative sea level during the process sequence. However, a fourth possible explanation is that the inset sediments represent increasingly distal or waning discharges through sub-channels incised into the main body of glaciofluvial sediments following local ice retreat, marginal isostatic uplift and a fall in relative sea level. In this scenario, as the ice margin retreated northward (Culleton, 1978; Synge, 1981) from the proposed ice margin in Zone 3, increasingly distal, waning flow regime sediments would have been deposited into the scour channels incised during initial stages of local crustal uplift following ice retreat, the inset units reflecting an aggradational phase following incision. Finally, notwithstanding which of these explanations for the channel stratigraphy, if any, is correct, rising eustatic sea level is likely to have exceeded the pace and quantity of isostatic uplift, explaining the aggradational phase and resulting in the submergence of the glaciofluvial channel and ultimately creating Waterford Harbour as it now appears.

Acknowledgements

The authors thank the following for their advice and assistance: the Master and crew of the RV Celtic Voyager, Sara Tubb, Mick Gillooly, Barry Kavanagh, Caitríona NicAonghusa, Rory Quinn, Peadar McArdle, Deepak Inamdar, Mick Geoghegan, Helen Gwinnutt, Bob Devoy and Andy Wheeler, Chris Bean, Stephen Hannon, Ken Blackmore, Martin Thorp, Arnold Horner, Joe Brady, Philip Allen and Alex Densmore. Submerged ice marginal forms in the Celtic Sea 165

References

CULLETON, E.B. (1978) Characterisation of the glacial deposits in south , Proceedings of the Royal Irish Academy, 78B, 20, 293-308. DEVOY, R.J.N. (1995) Deglaciation, Earth crustal behaviour and sea-level changes in the determination of insularity: a perspective from Ireland, In: Preece R.C. (ed.) Island Britain: a Quaternary perspective. Geological Society Special Publication, 96, 181-208. ENVIRONMENTAL PROTECTION AGENCY (1997) Hydrological data: a listing of water level recorders and summary statistics at selected gauging stations. Dublin: EPA. EYLES, N. and McCABE, A.M. (1989) The Late Devensian (22,000BP) Irish Sea Basin: the sedi- mentary record of a collapsed ice sheet margin, Quaternary Science Reviews, 8, 307-351. FARRINGTON, A. (1959) The Lee basin. Part one: glaciation, Proceedings of the Royal Irish Academy, 60, B, 3, 135-166. GALLAGHER, C. (1997) Alluvial Heavy Minerals as Indicators of Late Pleistocene Ice Flow in the Irish Midlands, Irish Geography, 30(1), 37-48. GALLAGHER, C. and THORP, M. (1997) The age of the Pleistocene raised beach near Fethard, , using Infra Red Stimulated Luminescence (IRSL), Irish Geography, 30(2), 68-89. GALLAGHER, C. (2002) The morphology and palaeohydrology of a submerged glaciofluvial channel emerging from Waterford Harbour onto the nearshore continental shelf of the Celtic Sea, Irish Geography, 35(2), 111-132. MAIZELS, J.K. and AITKEN, J. (1991) Palaeohydrological change during deglaciation in upland Britain: a case study from northeast Scotland, In: Starkel L., Gregory, K.J. and Thomas, J.B. (eds) Temperate Palaeohydrology. London: Wiley, 105-140. MARTIN, C.P. (1930) The raised beaches of the east coast of Ireland, Scientific Proceedings of the Royal Dublin Society, N.S. 19, 459-511. McCABE, A.M. (1993) Drumlin bedforms and related ice marginal depositional systems in Ireland, Irish Geography, 26(1), 22-44. McCABE, A.M. and CLARKE, P.U. (1998) Ice-sheet variability around the North Atlantic Ocean during the last deglaciation, Nature, 392, 373-377. McCABE, A.M., KNIGHT, J. and McCARRON, S. (1998) Evidence for Heinrich event 1 in the British Isles, Journal of Quaternary Science, 13, 6, 549-568. O’COFAIGH and EVANS, D. (2001) Sedimentary evidence for deforming bed conditions associated with a grounded Late Devensian Irish Sea glacier, southern Ireland, Journal of Quaternary Science, 16, 435-454. ORME, A.R. (1964) Planation surfaces in the Drum Hills, , and their wider implications, Irish Geography, 5(1), 48-72. ORME, A.R (1966) Quaternary changes of sea level, Transactions of the Institute of British Geographers, 39, 127-140. ROTNICKI, K. (1991) Retrodiction of Palaeodischarges of Meandering and Sinuous Alluvial Rivers and its Palaehydroclimatic Implications, In: Starkel L., Gregory, K.J. and Thomas, J.B. (eds) Temperate Palaeohydrology. London: Wiley, 431-470. STILLMAN, C.J. (1968), The post glacial change in sea level in southwestern Ireland: new evidence from fresh-water deposits on the floor of Bantry Bay, Scientific Proceedings of the Royal Dublin Society, A, 3, 11, 125-127. SYNGE, F.M. (1981) Quaternary glaciation and changes in sea level in the south of Ireland, Geologie en Mijnbouw, 60, 305-315. THOMAS, G.S.P. and SUMMERS, A.J. (1982) Drop-stone and allied structures from Pleistocene waterlain till at Ely House, county Wexford, Journal of Earth Sciences of the Royal Dublin Society, 4, 109-119. WRIGHT, W.B. and MUFF, H.B. (1904) The pre-glacial raised beach of the south coast of Ireland, Scientific Proceedings of the Royal Dublin Society, N.S. 10, 250-324.